What are the set of functions that are to be executed to get the following output?

[answerhappygod]

cat
a) push(c, s); push(a, s); push(t, s);
pop(s); pop(s); pop(s);
b) push(c,s); pop(s); push(a,s); pop(s);push(t,s);pop(s);
c) pop(c ); pop(a); pop(t);
d) push(c,s); push(a,s); pop(t);